Reduction of fuel consumption is one of the major technical issues in automobiles. As one of the means for this issue, demands for reduction of the weight of pneumatic tires have been strongly increased. On the other hand, an inner liner layer or other air permeation preventive layer comprised of a halogenated butyl rubber or other rubber having a low air permeability is conventionally provided at the inner surface of the pneumatic tire to maintain a constant tire air pressure.
However, a halogenated butyl rubber has a large hysteresis loss, and therefore, after vulcanization of the tire, when rippling is caused in the rubber of the inner surface of the carcass layer and the inner liner layer, since the inner liner layer deforms along with deformation of the carcass layer, there is the problem of an increase of the rolling resistance. Therefore, in general, a rubber sheet called a "tie rubber" having a small hysteresis loss has been inserted between the inner liner layer (for example, halogenated butyl rubber) and rubber of the inner surface of the carcass layer when joining the two. Accordingly, in addition to the thickness of the inner liner layer of the halogenated butyl rubber, the thickness of the tie rubber is added and the layer overall becomes a thickness of over 1 mm (i.e., 1,000 .mu.m). As a result, this has become a factor increasing the weight of the final tire.
Techniques have been proposed for using various materials in place of the butyl rubber or other low air permeability rubber as the inner liner layer of a pneumatic tire. For example, Japanese Examined Patent Publication (Kokoku) No. 47-31761 discloses to coat the inner surface of a vulcanized tire with, at a thickness of not more than 0.1 mm, a solution or dispersion of a synthetic resin such as a polyvinylidene chloride, saturated polyester resin, or polyamide resin having an air permeation coefficient cm.sup.3 (standard state)/cm.multidot.sec.multidot.mmHg! of not more than 10.times.10.sup.-13 at 30.degree. C. and not more than 50.times.10.sup.-13 at 70.degree. C.
Japanese Unexamined Patent Publication (Kokai) No. 5-330307 discloses to halogenate the inner surface of the tire (using a conventionally known chlorination solution, bromine solution, or iodine solution) and form, on the top thereof, a polymer film (the thickness=10 to 200 .mu.m) of methoxymethylated nylon, copolymer nylon, a blend of polyurethane and polyvinylidene chloride, or a blend of polyurethane and polyvinylidene fluoride.
Further, Japanese Unexamined Patent Publication (Kokai) No. 5-318618 discloses a pneumatic tire using a thin film of methoxymethylated nylon as an inner liner. According to this technique, a solution or emulsion of methoxymethylated nylon is spread or coated on the inner surface of a green tire, then the tire is vulcanized or a solution or emulsion of methoxymethylated nylon is spread or coated on the inner surface of the tire after vulcanization so as to produce the pneumatic tire.
Further, Japanese Unexamined Patent Publication (Kokai) No. 6-40207 is an example of using as an air permeation preventive layer composed of a tire a multilayer film having a low air permeability layer comprised of a polyvinylidene chloride film or ethylene-vinyl alcohol copolymer film and an adhesive layer comprised of a polyolefin film, aliphatic polyamide film, or polyurethane film. In this example, it is proposed to laminate the low air permeability layer and the adhesive layer to form a thin film and to laminate that thin film on the inner surface of a green tire comprised of unvulcanized rubber so that the adhesive layer contacts the carcass layer, then vulcanize and mold the green tire so as to provide an air permeation preventive layer on the inside of the tire. By using this multilayer film as the air permeation preventive layer, it is possible to reduce the thickness of the air permeation preventive layer from the past and to reduce the weight of the tire without detracting from the retention of the air pressure. However, when forming an air permeation preventive layer such as the inner liner from such a thermoplastic multilayer film, if the bond at the connecting portion (i.e., splice portion) of the multilayer film becomes poor, the splice portion of the liner of the final product may end up opening. Further, since the elongation of the splice portion is small at the time of inflation in the process for molding the tire, the uniformity of the tire is decreased and there is the possibility of occurrence of vibration when the automobile is being driven or a decline in the high speed durability.
Japanese Unexamined Patent Publication (Kokai) No. 59-93344 discloses an apparatus for forming a tire by adhering a cylindrical member of an inner liner etc. to the inside of a green tire. According to this apparatus, the cylindrical inner liner (i.e., cylindrical member) is continuously extruded from an extruder, inserted into the green tire carried on the tire table, and made to adhere to the inside of the green tire by inflation of a bladder. However, if it is attempted to use this apparatus when using a thermoplastic film as the inner liner material, when extruding the cylindrical film continuously downward, since the thermoplasticity is large, the cylindrical shape will not be held or the precision of the thickness of the film will not be obtained and the problem will arise of difficulty in cutting the cylindrical member (that is, the cylindrical thermoplastic film) inserted into the green tire by a cutter.
As explained above, various materials have been proposed for making the inner liner layer of a pneumatic tire in place of butyl rubber, but none has yet been commercialized. In view of this situation, we previously proposed, as a new material for an air permeation preventive layer, a polymer composition containing a thermoplastic resin or a blend of a thermoplastic resin and elastomer having an air permeation coefficient of not more than 25.times.10.sup.-2 cc.multidot.cm/cm.sup.2 .multidot.sec.multidot.cmHg and a Young's modulus of 1 to 500 MPa (for example, see specifications of Japanese Patent Application Nos. 7-150353, 7-28318, 7-28257, 7-26844, 7-28320, 7-11752, etc.)
However, when fabricating an air permeation preventive layer, for example, as shown in FIGS. 1(a) and (b), in the case of the conventional butyl rubber liner, when moving from the first molding step to the second molding step at the time of processing, the lift caused in the second molding step shown in FIG. 1(b) causes the connecting portion 3 of the film liner 2 at the bottom of the carcass layer 1 to elongate, and therefore, makes the tack of the film liner smaller, and therefore, the splice portion of the film liner ends up opening up (i.e., splice opening 4).
Further, as shown in FIG. 2, in a tire of a conventional single-ply carcass structure comprised of a carcass 5, inner liner layer (air permeation preventive layer) 6, and tie rubber 7, there is the problem of the occurrence of an air deposit A caused by the step difference of the splice portion (connecting portion) 8. To avoid this, the splice portion 8' of the inner liner layer 6/tie rubber 7 and splice portion 8" of the carcass 5 are offset in the circumferential direction as shown in FIG. 3 so as to keep the step difference of the splice portions small during the molding. However, with this molding method, there is a major problem in the productivity of the tire.